1. Field of the Invention
The invention relates to the field of commercial FM stereophonic broadcasting and, more specifically, to a method of amplitude and frequency limiting an FM composite modulation signal prior to application to the FM modulator stage of an FM broadcast transmitter.
2. The Prior Art
In commercial FM stereophonic broadcasting, it is desirable to maintain a high average modulation level. In addition, the Federal Communications Commission (FCC), which regulates all commercial broadcast stations, requires that a given FM broadcast signal be constrained to preset limits on both the amplitude and the frequency of its content. This constraint assures that any one signal does not occupy any more spectrum space than is necessary to provide normal service and to avoid possible interference to other users in the adjacent portions of the same spectrum.
More that just a simple data modulated waveform, the FM stereophonic composite signal contains a plurality of components.
The include the following:
1. Left channel information (L). PA1 2. Right channel information (R). PA1 3. Left plus right (or "L+R" or "Main Channel) information (the sum of the left and right channels). PA1 4. Left minus right ("L-R" or "sub channel" information) (the difference between the left and right channels. PA1 5. 19 kilohertz "Pilot" tone sine wave. PA1 6. Optional modulated fixed-frequency subcarriers from 57 to 99 kilohertz used for background music, digital station identification and paging, and etc. These subcarriers are also referred to as SCA's, as in Subsidiary Communications Authorizations.
Referring to FIG. 1, the composite FM stereophonic signal consists of a main channel and a sub channel. The main channel is modulated by the sum of the left and the right stereo signals (L+R). The sub channel consists of the two side bands (upper and lower) of a 38 kilohertz suppressed carrier that is modulated with a difference of the left and right channels (L-R). Since the modulating frequencies of the individual left and right channels can be as high as 15 kilohertz, the L-R sidebands occupy the baseband spectrum from 23 kilohertz to 53 kilohertz.
The spectrum also includes a pilot carrier of 19 kilohertz. In that the second harmonic of the 19 kilohertz pilot is a 38 khz signal (corresponding in phase to the suppressed carrier used to modulate the FM subcarriers), a stereo receiver can use the phase-correct 38 khz generated signal to properly demodulate the transmitted L-R sideband signals. This demodulation is dependent on the quality of the received 19 khz signal. Hence, for correct demodulation, the stereo broadcasting components must preserve the integrity of the 19 khz pilot signal. Unfortunately, common signal processing steps used to prepare a complex FM signal for broadcast (which include frequency and amplitude limiting) deteriorate the integrity of the 19 khz pilot signal and make decoding of the sidebands difficult to impossible. Furthermore, after-broadcast interference may deteriorate the quality of the received complex FM signal further making it even more difficult to recreate the 38 khz carrier and, thus, properly demodulate the modulated sidebands. Because the quality of broadcast signal can only degrade after broadcasting, steps must be taken to ensure the highest quality signal including the 19 khz pilot is broadcast.
As to the additional components of the complex FM signal, the subsidiary communications authorizations (or SCA'S) permit the optional addition of one or more subcarriers to the composite signal. These subcarriers may be used for the transmission of voice, music, or data signals related or unrelated to FM broadcast station operation.
As stated above, a signal's amplitude and bandwidth consumed must be limited to conform with FCC regulations. One way of limiting the amplitude of a signal includes passing the signal through a clipper circuit. The portion of signal above the clipper's threshold is considered "overshoot" and is eliminated. In addition to complying with FCC amplitude regulations, passing the signal through a clipper has the added benefit of making the signal sound louder to a listener. While limiting the amplitude of an inputted signal, the clipper, however, introduces unwanted transients in to the signal because of the sharp cutoff of the overshoot. Represented by additional higher order harmonics, these transients increase the bandwidth consumed by the signal. Because the frequencies corresponding to the transients fall outside the band constraints as imposed by the FCC, these frequencies must be eliminated. These higher order harmonics appear as "ringing" on the clipped waveform. In addition to clipping, other signals introduce transients. These signals include sharp waves and, in general, signals with fast response times. So while going through the trouble to band limit a signal before the clipping stage, one commonly introduces unwanted out-of-band products into the signal in the clipping stage. While the signal appears louder, the excess products decrease the signal-to-distortion ratio as well as cause the broadcast signal to violate FCC regulations.
Common in the prior art is a clipper including diodes as shown in FIG. 6. While a diode-based clipper can be effective, it has its drawbacks. Diodes are not ideal. Diodes exhibit a short delay between not conducting and conducting. Also, the slow turn-on of the diodes allows extra distortion to creep into the signal being transmitted as well. So, even if a signal is not being clipped, but is just below the turn-on voltage of the diodes, the post-clipper signal can still include distortion because of the partial turn-on of the diodes near the peaks of the inputted signal. Furthermore, while one might use higher quality diodes including higher speed schottky diodes, the detrimental effects of the diodes are minimized but not eliminated. Accordingly, a diode-based clipper cannot cleanly maximize the amplitude of complex signal.
One approach to limiting the excess frequencies introduced into a composite signal by clipping includes low-pass filtering the signal before transmission. One known method included adding a low-pass filter to the output of a clipper with the filter followed by another clipper. Unfortunately in this prior art system, each element introduced additional unwanted artifacts into the complex signal. Another prior art reference, namely U.S. Pat. No. 4,460,871 to Orban, discloses multiple clippers and low-pass filters. However, it's approach the above problems is complicated by using a cross-over network and partial initial clipping. It also appears to fail to adequately protect the 19 khz pilot signal.
Thus, in view of the above, the prior art's method of protecting the 19 khz pilot carrier and minimizing introduced distortion while complying with FCC regulations remains inadequate.